1. Field of the Invention
The present invention relates to an apparatus for stabilizing soil and reducing the possibility of structural damage to foundations used to support buildings and dwellings. More particularly, this invention relates to an apparatus for controlling soil moisture content to stabilize forces being exerted against foundations by soil which expands and contracts in relation to its moisture content.
2. Description of the Prior Art
The expansion and contraction of clay soils has resulted in billions of dollars of damage to building foundations. Soils containing clay expand and contract as moisture content changes. Soils with a high content of certain clays can shrink to half their original volume as they relinquish water and dry out from their saturated state. A foundation constructed on those types of soil will experience varying structural loads when the soil expands and contracts. In geographical areas with a wide variation in seasonal precipitation, soil expansion and contraction will cause bending forces in a foundation that cause damage and possibly lead to structural failure.
Another problem occurs when one section of the soil underneath the foundation experiences localized moisture deprivation. Localized depletion is created by the existence of vegetation around a foundation. For example, the roots of a tree present near the foundation will absorb moisture from that specific area causing a localized depletion of soil moisture content. When that occurs, the soil contracts causing that particular foundation section to sag. That, in turn, creates unequal load stress about the entire foundation resulting in structural failure. Traditionally, piers have been installed after structural damage to prevent the foundation from further movement. However, in many instances piers may not be a permanent solution, and they are costly to the homeowner.
Systems have been developed which attempt to maintain the soil at a constant level of moisture. The aim is to prevent wet-dry cycles and thereby prevent the volume changes in soil that cause foundation damage. One such system is disclosed in U.S. Pat. No. 4,534,143 issued to Goines et al. The system of Goines et al. operates to supply water to the soil surrounding a foundation to produce a stable soil moisture level and prevent foundation stress. However, the fact that the Goines et al. system can only add water in preset amounts and at preset times is a serious drawback. It will continue to add water during rainy periods and can worsen the puddling of water around a foundation. Conversely, when hot, dry periods occur, the preset water is inadequate to stabilize the moisture content which can lead to serious soil shrinkage and foundation damage. Furthermore, the Goines et al. system cannot compensate for localized moisture depletion as might be caused by a large tree. The overlying foundation can experience a downward deflection into the localized area of decreased support and damage a foundation despite the presence of the functioning watering system. Even at its best, the Goines et al. system demands sound judgment about weather and its affects causing frequent adjustment by the system's owner.
An improvement over the Goines et al. system is disclosed in U.S. Pat. No. 4,878,781 issued to Gregory et al. The Gregory et al. system addresses the problem of seasonal changes by installing a flow regulator preset to a relatively high flow of water during hot and dry seasons and a relatively low flow of water for cooler and less dry seasons. However, the Gregory et al. system provides only for seasonal changes and still relies upon human judgment and frequent resetting for foundation protection. As with Goines et al., hazards remain from the potential for too much or too little water.
Another system that addresses the problem of localized soil moisture depletion is disclosed in U.S. Pat. No. 4,879,852 issued to Tripp. That system provides water to the soil underneath the foundation on a demand basis and also provides for a localized dispersion of water. Additional water can, therefore, be supplied to those areas that are lacking, such as those near plants and vegetation, without wasting water on those areas sufficiently hydrated. The Tripp system uses a series of moisture sensors placed beneath the surface of the soil to determine the localized water depletion. A control box containing an electronic processor located near the foundation receives and processes the signals from the moisture content sensors. After the moisture content of various areas around the foundation has been determined, water is introduced into those areas based upon the amount of dehydration. The electronic processor controls various sets of control valves to allow water to flow to each of the areas until the selected water content of that area has been met. The control valves are then closed by the electronic processor until water is again needed.
Although the Tripp system is said to be more effective than previous systems, it will not be in clay-based soils. In clay, conventional moisture content sensors are subject to serious measurement inaccuracies, often greater than plus or minus 50%. These occur because most conventional moisture content sensors measure the dielectric constant of the water in comparison to the dielectric constant of the surrounding soil in order to determine the overall moisture content of the soil. Specifically, measurement inaccuracies in clay occur because the dielectric constant of water is approximately 80 and the dielectric constant of clay ranges in the magnitude of 10.sup.6 through 10.sup.7. Determining changes in the dielectric constant of water as measured against the dynamic range of the dielectric constant of clay is difficult and prone to produce inaccurate results. The available technology for the precise moisture measurement in clay is cost-prohibitive to most homeowners. The Tripp system, therefore, is subject to inherent errors in measuring the moisture content of the soil that can cause either excessive watering of a localized area, erosion or underwatering which produces the localized foundational stress that causes structural damage.
The present invention overcomes those problems and other problems by replacing the moisture content sensors used in conventional foundation stabilization systems with specialized stress sensors. The sensors of the present invention are specifically designed to measure foundation stress resulting from the expansion or contraction of underlying soil based on moisture content. The system of the present invention introduces water into either all of the surrounding soil or specifically into localized areas until the force exerted on the foundation is equalized and at the proper level. The stress sensors of the present invention provide a much more accurate means of controlling soil movement. The prevention of damaging soil movement beneath a foundation, and the maintenance of soil stability when the foundation is positioned in a desirable manner are the ultimate aims of a foundation watering system. The present invention delivers into foundation soil variable amounts of water in a quantity sufficient to maintain the desired foundation alignment. In so doing, the problems of moisture measurement in soil and the complexities of weather prediction are bypassed. Highly precise strain gauges are placed at various locations about a foundation to sense foundation loads. In response to changes in foundation stress as measured by the strain gauges, water is precisely delivered to the various locations in order to maintain ideal loads.